This invention relates generally to elements such as uniaxial crystals, which are used to separate a non-collimated input beam into two orthogonally polarized sub-beams or to combine two orthogonally polarized beams into a single beam.
Polarization independent devices such as optical circulators and isolators generally require separating the input beam having an unknown polarization state, into two orthogonally polarized sub-beams. These sub-beams are routed through the isolating elements of the device such as reciprocal and non-reciprocal rotators and are combined at an output end. However, if the beams are launched in a backwards direction non-reciprocal elements ensure that the light does not couple back into the input port. Rutile crystals are well known for the purpose of separating an input beam into two orthogonally polarized sub-beams thereby serving as a polarization beam splitter, or operated in an opposite direction as a polarization beam combiner. Within this specification the term polarization beam splitter is used however it should be understood, that the same device can serve as a polarization beam combiner operated in reverse.
It has been typical, for light propagating within these crystals to be collimated, most often by a graded index (GRIN) lens. In this instance a relatively large crystal is required to ensure separation of two beams that have diameters typically as large as 350 xcexcm. However, recently, it was discovered that very small crystals, about {fraction (1/50)}th the size of conventional crystals could be used with a non-collimated beam; using such small crystals substantially lessens the cost of manufacturing an optical device.
Many polarizers and polarizing beam splitters that separate an input from a beam into two orthogonal polarizations or that combine two orthogonal polarizations into one output beam are known to the art. These include a Glan-Thompson polarizer, which is a block of birefringent material cut into prisms and then cemented together, that acts by reflecting one polarization component at the cement interface and by transmitting the other. Another polarizer is a Glan-Taylor polarizer that is similar to the Glan-Thompson polarizer but uses an air space instead of cement to separate polarization components. The Wollaston, Rochon and Senarmont beam-splitters separate polarization components by transmitting the components through an interface.
However, one disadvantage of all of these prior art polarization beam splitters/combiners is that there is a difference in optical path length for the two separated orthogonal polarizations traveling through a birefringent crystal. Using birefringent crystals where the light propagating therethrough is not collimated, leads to an increase in insertion loss due to a defocusing or a need to compensate for the path length difference. The sub-beams follow a slow axis and a fast axis, which corresponds to this difference in optical path length shown in FIG. 1. In this figure an optical fibre 10 is shown having its end optically coupled with a rutile crystal 22 via a lens 12. It is typical after separating the beam into its two orthogonal polarization states to couple the light into two fibre ends (not shown). However, as can be seen from FIG. 1, the two focus spots do not lie on a same focal plane. This is due to the optical path length difference for the e-ray and the o-ray through the crystal 22. Generally pairs of optical fibres are held securely in a fixed manner in an optical fibre tube. In this instance if such a tube was used and disposed at one of the spots 14a or 14b, the other of the spots would not be in focus at the tube end, and light from either the e-ray or o-ray path would couple poorly.
It is an object of this invention to provide a device, which lessens or obviates this optical path length difference, or which provides compensation for PMD in an optical device.
It is an object of this invention to provide a polarization beam splitter/combiner that has substantially same optical path lengths for two split or combined non-collimated beams propagating therethrough.
Alternatively, it is an object of this invention to provide a polarization beam splitter/combiner that provides a selected optical path length difference for TE and TM polarization modes propagating therethrough.
An application for a beam splitter/combiner having equalized path length is found in integrated optical chromatic spatial dispersive elements, where polarization effects cause unwanted losses. There are three effects in a chromatic spatial dispersive element that are influenced by the light polarization state. These are polarization dependent wavelength (PD xcex), polarization mode dispersion (PMD), and polarization dependent loss (PDL).
Ando et al. in U.S. Pat. No. 5,901,259 assigned to Nippon Telegraph and Telephone Corporation, propose inserting a polyimide optical waveplate in the middle of the optical path of an optical waveguide device, in order to reduce the polarization dependence of a planar lightwave circuit. However, this is difficult to manufacture and introduces undesired losses in the device.
The polarization beam splitter/combiner in accordance with the present invention can be used at the input or output of an optical chromatic spatial dispersive element to overcome these polarization effects without difficult manufacturing techniques or the introduction of unacceptable losses.
Thus, it is a further object of the present invention to provide a polarization beam splitter/combiner which compensates for polarization effects within an optical chromatic spatial dispersive element.
In accordance with the invention there is provided, a polarization beam splitter/combiner for splitting a non-collimated beam of light into first and second beams of orthogonal polarization, and for combining first and second beams of light of orthogonal polarization into a beam of light comprising:
a first port for launching a beam of light into the polarization beam splitter/combiner in a forward direction or for receiving a beam of light from the polarization beam splitter/combiner in a reverse direction;
a first uniaxial crystal having an o-ray path and an e-ray path and having the first port optically coupled to an end face thereof;
a second uniaxial crystal having an e-ray path and an o-ray path such that the e-ray path of the second uniaxial crystal is optically coupled with the o-ray path of the first uniaxial crystal and the o-ray path of the second uniaxial crystal is optically coupled with the e-ray path of the first uniaxial crystal; and
a second and a third port optically coupled to an end face of the second uniaxial crystal for one of outputting a first beam of a first polarization state and a second beam of a second orthogonal polarization state in the forward direction and for launching the first beam of the first polarization state and the second beam of the second orthogonal polarization state into the polarization beam splitter/combiner in the reverse direction, wherein the polarization beam splitter/combiner provides selected relative optical path lengths for a first beam of the first polarization state and a second beam of the second orthogonal polarization state propagating therethrough.
In accordance with the invention it is alternatively provided, wherein an axis of the second crystal is aligned in such a manner that the o-ray path is retarded by an extraordinary index of refraction of the crystal and the e-ray path is retarded by an ordinary index of refraction to determine the relative optical path lengths.
A further embodiment, in accordance with the present invention, including a polarization rotator between the first uniaxial crystal and the second uniaxial crystal for rotating the polarization of light received from the first uniaxial crystal in a forward direction or for rotating the polarization of light received from the second uniaxial crystal in a reverse direction.
In a still further embodiment of the present invention, the polarization beam splitter/combiner including a polarizer for compensating for polarization dependent loss in an optical device.
Alternatively, in accordance with a preferred embodiment, a polarization beam splitter/combiner for providing polarization compensation in an optical chromatic spatial dispersion element comprises:
a first birefringent beam splitting element having at least one single port on an endface thereof;
a second birefringent element optically coupled to the first birefringent element, having at least one pair of ports spaced apart on an endface thereof associated with the at least one single port, and
a first optical path from the at least one single port to one port of the associated pair of ports and a second optical path from the at least one single port to another port of the associated pair of ports; and
means for changing the effective polarization state of a non-collimated beam of light propagating from the first birefringent element to the second birefringent element, such that the first optical path and the second optical path have relative path lengths selected to compensate for a known polarization mode dispersion in the optical chromatic spatial dispersion element.